mc9s12xd256 Freescale Semiconductor, Inc, mc9s12xd256 Datasheet - Page 531

• Manufacturer Part Number: mc9s12xd256
• Description: Hcs12x Microcontrollers
• Manufacturer: Freescale Semiconductor, Inc

12.4.3
During an SPI transmission, data is transmitted (shifted out serially) and received (shifted in serially)
simultaneously. The serial clock (SCK) synchronizes shifting and sampling of the information on the two
serial data lines. A slave select line allows selection of an individual slave SPI device; slave devices that
are not selected do not interfere with SPI bus activities. Optionally, on a master SPI device, the slave select
line can be used to indicate multiple-master bus contention.
12.4.3.1
Using two bits in the SPI control register 1, software selects one of four combinations of serial clock phase
and polarity.
The CPOL clock polarity control bit specifies an active high or low clock and has no significant effect on
the transmission format.
The CPHA clock phase control bit selects one of two fundamentally different transmission formats.
Clock phase and polarity should be identical for the master SPI device and the communicating slave
device. In some cases, the phase and polarity are changed between transmissions to allow a master device
to communicate with peripheral slaves having different requirements.
12.4.3.2
The first edge on the SCK line is used to clock the first data bit of the slave into the master and the first
data bit of the master into the slave. In some peripherals, the first bit of the slave’s data is available at the
slave’s data out pin as soon as the slave is selected. In this format, the first SCK edge is issued a half cycle
after SS has become low.
A half SCK cycle later, the second edge appears on the SCK line. When this second edge occurs, the value
previously latched from the serial data input pin is shifted into the LSB or MSB of the shift register,
depending on LSBFE bit.
After this second edge, the next bit of the SPI master data is transmitted out of the serial data output pin of
the master to the serial input pin on the slave. This process continues for a total of 16 edges on the SCK
line, with data being latched on odd numbered edges and shifted on even numbered edges.
Freescale Semiconductor
Transmission Formats
Clock Phase and Polarity Controls
CPHA = 0 Transfer Format
SHIFT REGISTER
GENERATOR
BAUD RATE
MASTER SPI
Figure 12-10. Master/Slave Transfer Block Diagram
MC9S12XDP512 Data Sheet, Rev. 2.17
MISO
MOSI
SCK
SS
V
DD
MISO
MOSI
SCK
SS
Chapter 12 Serial Peripheral Interface (S12SPIV4)
SHIFT REGISTER
SLAVE SPI
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